Elucidating effects of form factors on thermal and aging behavior of cylindrical lithium-ion batteries

Abstract

Cylindrical lithium-ion cells are commercially available in different form factors, e.g., 18650, 21700, 26650, 32700 and 4680. The larger 4680 cells are gaining popularity, especially after the patent filing by Tesla. This paper aims to perform a comparative analysis of the thermal and aging behavior of different form factors using NMC–graphite chemistry during prolonged cycling at moderate operating conditions. A physics-based aging model for lithium-ion batteries is introduced, incorporating side reactions such as solid electrolyte interface (SEI) formation, SEI reformation due to graphite layer cracking, and lithium plating. During cycling, the cell temperature increases with time for all form factors due to internal heat generation. Notably, larger form factor cells experience higher temperatures, attributed to their inferior surface-to-volume ratio for heat dissipation, thereby accelerating both SEI formation and triggering lithium plating in subsequent cycles. This accelerated aging manifests in increased heat generation and prolonged charging for larger cells. The 4680 cell showed the highest rate of capacity loss with a 20% reduction over 480 cycles as compared to the 600 cycles of the 18650 cell. Highlighting accelerated aging in the larger cells due to suboptimal heat dissipation, this study underscores the pressing need for advanced thermal management strategies tailored to the unique challenges posed by larger cells.